Start-up circuit for a power adapter

ABSTRACT

A start-up circuit for a power adapter and method of operating the same. In one embodiment, the power adapter includes a start-up circuit configured to provide an initial bias voltage for the power adapter. The power adapter also includes a crowbar circuit configured to turn on the start-up circuit upon loss of an ac mains voltage supplied to the power adapter.

This application is a continuation in part of, and claims priority to,U.S. patent application Ser. No. 12/486,493, entitled “Power AdapterEmploying a Power Reducer,” filed on Jun. 17, 2009, which isincorporated herein by reference.

TECHNICAL FIELD

The present invention is directed, in general, to power electronics and,more specifically, to a start-up circuit for a power adapter and methodof operating the same.

BACKGROUND

A switched-mode power converter (also referred to as a “power converter”or “regulator”) is a power supply or power processing circuit thatconverts an input voltage waveform into a specified output voltagewaveform. DC-DC power converters convert a direct current (“dc”) inputvoltage into a dc output voltage. Controllers associated with the powerconverters manage an operation thereof by controlling conduction periodsof power switches employed therein. Generally, the controllers arecoupled between an input and output of the power converter in a feedbackloop configuration (also referred to as a “control loop” or “closedcontrol loop”).

Typically, the controller measures an output characteristic (e.g., anoutput voltage, an output current, or a combination of an output voltageand an output current) of the power converter, and based thereonmodifies a duty cycle of a power switch of the power converter. The dutycycle “D” is a ratio represented by a conduction period of a powerswitch to a switching period thereof. Thus, if a power switch conductsfor half of the switching period, the duty cycle for the power switchwould be 0.5 (or 50 percent). Additionally, as the voltage or thecurrent for systems, such as a microprocessor powered by the powerconverter, dynamically change (e.g., as a computational load on themicroprocessor changes), the controller should be configured todynamically increase or decrease the duty cycle of the power switchestherein to maintain an output characteristic such as an output voltageat a desired value.

A power converter with a low power rating designed to convert analternating current (“ac”) mains voltage to a regulated dc outputvoltage to power an electronic load such as a printer, modem, orpersonal computer is generally referred to as an “ac power adapter” or a“power adapter,” or, herein succinctly, as an “adapter.” Industrystandards have required continual reductions in no-load power supplyloss to reduce power consumed by millions of power adapters that remainplugged in, but are not in use. Efficiency requirements at very lowoutput power levels were established in view of the typical loadpresented by an electronic device in an idle or sleep mode, which is anoperational state for a large fraction of the time for such devices in ahome or office environment.

No-load power loss of a power adapter is typically dominated by threephenomena. The first phenomenon is directed to the current drawn fromhigh-voltage supply bus to provide power to the controller of theadapter. The high-voltage power draw is sometimes shut off when theadapter is in operation and power can be supplied from an auxiliarywinding of a transformer thereof. However, in the absence of operationof the adapter (e.g., a start-up condition or in the event of completeshutdown of the controller), the high-voltage supply bus provides powerto the controller directly. While the current required to start ormaintain controller operation may be small, the fact that it comes froma high-voltage bus causes a higher-than-optimal draw of power from theinput of the adapter.

The second phenomenon is directed to the current flow in a bleederresistor coupled across an “X-capacitor” (i.e., a safety ratedcapacitor) of the adapter. An X-capacitor is a capacitor coupled acrossthe ac input power mains (also referred to as “ac mains”) to a powerconverter to reduce electromagnetic interference (“EMI”) produced by thepower converter and conducted back to the ac mains. A “Y-capacitor”(i.e., a safety rated capacitor) is an EMI-reducing capacitor coupledbetween ac mains to a power converter and an input-side groundingconductor. Both the X-capacitor and the Y-capacitor are distinguished bya safety voltage rating related to a peak voltage that the respectivecapacitor is required to sustain. Upon disconnection from the ac mains,the X-capacitor should be bled down to a low voltage in a short periodof time. Bleeding down an X-capacitor voltage is typically accomplishedwith a bleeder resistor coupled across the capacitor.

The third phenomenon is directed to gate drive and other continuingpower losses that do not vary with load. The third phenomenon iscommonly addressed by using a burst-mode of operation, wherein thecontroller is disabled for a period of time (e.g., one second) followedby a short pulse of high-power operation (e.g., 10 milliseconds (“ms”)),to provide a low average output power. The second phenomenon is commonlyaddressed by reducing generated EMI in various ways allowing a reductionin the size of the X-capacitor, which enables reduction of the bleederresistor current. The first phenomenon above is not usually addressed.

Even when the controller is disabled, it still draws a small butsignificant amount of power. Furthermore, the bleeder resistor coupledin parallel with the X-capacitor draws continuous power regardless ofload level. While the X-capacitor size can be reduced somewhat by goodEMI design practices, all adapters require at least a small X-capacitorto meet EMI requirements, resulting in bleeder resistor losses at noload.

These two types of power losses, while relatively small, have now becomesubstantial hindrances to lowering no-load losses as industryrequirements become stricter each year. Thus, despite the development ofnumerous strategies to reduce power losses of power adapters, nostrategy has emerged to provide substantial reduction of powerdissipation while an adapter provides minimal or no power to a load.Accordingly, what is needed in the art is a design approach and relatedmethod for a power adapter that enable further reduction of powerconverter losses without compromising end-product performance, and thatcan be advantageously adapted to high-volume manufacturing techniques.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention, including a start-up circuit for a poweradapter and method of operating the same. In one embodiment, the poweradapter includes a start-up circuit configured to provide an initialbias voltage for the power adapter. The power adapter also includes acrowbar circuit configured to turn on the start-up circuit upon loss ofan ac mains voltage supplied to the power adapter.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a schematic diagram of an embodiment of a poweradapter constructed according to the principles of the presentinvention;

FIG. 2 illustrates a schematic drawing of a portion of an embodiment ofa power adapter constructed according to the principles of the presentinvention;

FIG. 3 illustrates a graph showing selected voltages of the poweradapter of FIG. 2;

FIG. 4 illustrates a schematic drawing of a portion of an embodiment ofa power adapter constructed according to the principles of the presentinvention;

FIGS. 5, 6A and 6B illustrate graphs showing selected voltages of thepower adapter of FIG. 4; and

FIG. 7 illustrates a schematic drawing of an embodiment of portions of apower adapter formed with a start-up circuit and a crowbar circuitconfigured constructed according to the principles of the presentinvention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated, and may not beredescribed in the interest of brevity after the first instance. TheFIGUREs are drawn to illustrate the relevant aspects of exemplaryembodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present exemplary embodiments are discussedin detail below. It should be appreciated, however, that the presentinvention provides many applicable inventive concepts that can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to exemplaryembodiments in a specific context, namely, a start-up circuit for apower adapter. While the principles of the present invention will bedescribed in the environment of a power adapter, any application thatmay benefit from a power conversion device including a start-up circuitsuch as a power amplifier or a motor controller is well within the broadscope of the present invention.

A flyback power converter is frequently employed in low powerapplications such as a power adapter for a printer because of itssimplicity and low cost. The power adapters employing a flyback powerconverter are typically designed to operate continuously at a highoutput power level. Recalling that the loads presented to power adapterssuch as loads provided by printers and personal computers are generallyvariable and usually do not operate for an extended period of time at amaximum power level, a consideration for the design of power adaptersfor such applications is power conversion efficiency at no load and atlight loads.

Turning now to FIG. 1, illustrated is a schematic diagram of anembodiment of a power adapter constructed according to the principles ofthe present invention. A power train (e.g., a flyback power train) ofthe power converter (also referred to as a “flyback power converter”)includes a power switch Q_(main) coupled to an ac mains 110, anelectromagnetic interference (“EMI”) filter 120 that may include anX-capacitor and a Y-capacitor as described further hereinbelow, a bridgerectifier 130, and an input filter capacitor C_(in) to provide asubstantially filtered dc input voltage V_(in) to a magnetic device(e.g., an isolating transformer or transformer T₁). Although the EMIfilter 120 illustrated in FIG. 1 is positioned between the ac mains 110and the bridge rectifier 130, the EMI filter 120 may contain filteringcomponents positioned between the bridge rectifier 130 and thetransformer T₁. The transformer T₁ has a primary winding N_(p) and asecondary winding N_(s) with a turns ratio n:1 that is selected toprovide an output voltage V_(out) with consideration of a resulting dutycycle and stress on power train components.

The power switch Q_(main) (e.g., an n-channel field-effect transistor)is controlled by a pulse-width modulator (“PWM”) controller 140 thatcontrols the power switch Q_(main) to be conducting for a duty cycle.The power switch Q_(main) conducts in response to gate drive signalV_(G) produced by the PWM controller 140 with a switching frequency(often designated as “f_(s)”). The duty cycle is controlled (e.g.,adjusted) by the PWM controller 140 to regulate an output characteristicof the power converter such as an output voltage V_(out), an outputcurrent I_(out), or a combination thereof. A feedback path 150 enablesthe PWM controller 140 to control the duty cycle to regulate the outputcharacteristic of the power converter. Of course, as is well known inthe art, a circuit isolation element such as an opto-isolator may beemployed in the feedback path 150 to maintain input-output isolation ofthe power converter. The ac voltage appearing on the secondary windingN_(s) of the transformer T₁ is rectified by the diode D₁, and the dccomponent of the resulting waveform is coupled to the output through thelow-pass output filter including an output filter capacitor C_(out) toproduce the output voltage V_(out).

During a first portion of the duty cycle, a current I_(pri) (e.g., aninductor current) flowing through the primary winding N_(p) of thetransformer T₁ increases as current flows from the input through thepower switch Q_(main). During a complementary portion of the duty cycle(generally co-existent with a complementary duty cycle 1-D of the powerswitch Q_(main)), the power switch Q_(main) is transitioned to anon-conducting state. Residual magnetic energy stored in the transformerT₁ causes conduction of current through the diode D₁ when the powerswitch Q_(main) is off. The diode D₁, which is coupled to the outputfilter capacitor C_(out), provides a path to maintain continuity of amagnetizing current of the transformer T₁. During the complementaryportion of the duty cycle, the magnetizing current flowing through thesecondary winding N_(s) of the transformer T₁ decreases. In general, theduty cycle of the power switch Q_(main) may be controlled (e.g.,adjusted) to maintain a regulation of or regulate the output voltageV_(out) of the power converter.

In order to regulate the output voltage V_(out), a value or a scaledvalue of the output voltage V_(out) is typically compared with areference voltage in the PWM controller 140 using an error amplifier(not shown) to control the duty cycle. This forms a negative feedbackarrangement to regulate the output voltage V_(out) to a (scaled) valueof the reference voltage. A larger duty cycle implies that the powerswitch Q_(main) is closed for a longer fraction of the switching periodof the power converter.

As introduced herein, no-load (also referred to as light or reducedload) losses of a power adapter are addressed. In one embodiment, apower converter of the power adapter (i.e., power conversion circuitrycoupled to the dc side of a bridge rectifier that rectifies an ac mainsvoltage) is substantially disconnected when the adapter receives asignal from an external source such as from a load coupled to theadapter. For example, a personal computer may transmit a signal to theadapter indicating that the adapter should enter a no-load operationalcondition. In response to the signal, the adapter disconnects the powerconverter of the adapter by opening a switch such as a metal-oxidesemiconductor field-effect transistor (“MOSFET”). Disconnecting thepower converter of the adapter removes substantial losses associatedwith the adapter other than losses that may be incurred by a bleederresistor for an X-capacitor.

In another embodiment, an active bleeder is coupled across anX-capacitor of a power adapter. When an ac mains voltage is supplied tothe adapter, the active bleeder is turned off. When the ac mains voltageis turned off, the active bleeder is turned on to quickly discharge theX-capacitor. Using an active bleeder removes a substantial portion ofthe losses incurred by the bleeder resistor for the X-capacitor.

As introduced herein, an active bleeder senses discharge of a capacitorsuch as a high-voltage capacitor that signals the presence of the acmains voltage. Discharge of the capacitor occurs when the ac mainsvoltage is removed from ac mains coupled to the adapter. The poweradapter may sense a removal of an ac mains voltage via pulsedetector(s), such as a pulse detector responsive to a fundamentalfrequency of the ac mains voltage to the adapter or a harmonic thereof.Each pulse detector is coupled to a respective rectifier diode that inturn is coupled to an ac mains terminal. Loss of the pulse trainproduced by either diode coupled to the ac mains terminal is taken as anindication of removal of the ac mains voltage to the adapter, and anactive bleeder (e.g., a bleeder MOSFET) coupled across an X-capacitor isturned on. Each of the two diodes produces its own pulse train inresponse to the ac mains voltage. A single charged X-capacitor may notby itself produce a reliable signal indicative of presence of an acmains voltage. By requiring both diodes to produce its respective pulsetrain, the circuit requires both the hot and neutral terminals toregularly have a positive high voltage to keep the pulse trains active,which can only happen when the ac mains voltage switches polarity on aregular basis (i.e., a false signal is not produced by presence of a dcvoltage on a capacitor to indicate presence of the ac mains voltage).Thus, a detection circuit for an active bleeder may be configured todetect loss of ac mains voltage employing a plurality of diodes adaptedto sense both polarities of the ac mains voltage.

Turning now to FIG. 2, illustrated is a schematic drawing of a portionof an embodiment of a power adapter constructed according to theprinciples of the present invention. The power adapter includes adisconnect switch Q2 that removes substantially all losses associatedwith the power adapter other than losses incurred by a bleeder resistorR1 associated with a capacitor (e.g. an X-capacitor C3). A resistor R3represents the power drain of an isolated power converter coupled to abridge rectifier 101, including its no-load drain, and a load coupled tothe power converter, such as a personal computer including a battery.Thus, even if the personal computer is disabled or disconnected from theadapter, the resistor R3 represents a positive power drain due to thehigh-frequency switching action of the power converter in the poweradapter as well as active control components in the power converter.

The disconnect switch (e.g., MOSFET) Q2 is the disconnect switch for theresistor R3. A pair of diodes D4, D5 charge a high-voltage capacitor C1to the peak of the ac mains voltage V1. The bleeder resistor R1 and azener diode Z1 produce a 12 volt source coupled to the gate of thedisconnect switch Q2 that turns on the disconnect switch Q2 in responseto the ac mains voltage V1 to the adapter.

When the load such as the personal computer enters a no-load orshut-down operational mode, it produces a secondary-side (i.e.,load-side) signal V2 at a high level, such as greater than three volts,to signal turn-off of a switch (e.g., MOSFET) Q3. Turning off the switchQ3 by the secondary-side signal V2 coupled to an inverter INV1 enablesoperation of an oscillator (e.g., a relaxation oscillator 102) formed byan inverting Schmitt trigger U1, a resistor R4 and a capacitor C2 (e.g.,a 10 nanofarad capacitor). The inverting Schmitt trigger U1 produces ahigh output signal in response to a low input signal. The secondary-siderelaxation oscillator 102 when it is enabled by turning off the switchQ3 produces a 3.3 volt square wave at its output at circuit node N1,which passes across the high-voltage boundary between primary andsecondary grounds of the adapter (wherein the primary and secondarygrounds are indicated by a “p” or “s” adjacent to a ground symbol)through a Y-capacitor CY. A voltage doubler 103 then boosts andrectifies this square-wave voltage to a dc level of about 6.6 volts. The6.6 volt dc level turns on a power switch (e.g., a MOSFET) Q1 whichshorts the 12 volt source (across the zener diode Z1) coupled to thegate of the disconnect switch Q2, thereby turning off the disconnectswitch Q2. The voltage doubler 103 may be omitted if the square waveproduced at the circuit node N1 is of sufficient amplitude to turn onthe power switch Q1. The inverter INV1 and the inverting Schmitt triggerU1 are coupled to an independent power source of the load (e.g., abattery V3 of a personal computer).

Although the power adapter illustrated in FIG. 2 and that illustratedand described herein later with reference to FIG. 4 are constructed witha signal-coupling capacitor (e.g., Y-capacitor CY) to transmit a signalacross the isolation boundary between primary- and secondary-sidegrounds, in an alternative embodiment, another isolation component suchas a pulse transformer or an opto-isolator may be employed, as is wellknown in the art, to transmit the square-wave signal (or other waveform)across the isolation boundary. The Y-capacitor CY illustrated in FIGS. 2and 4 provides a low-cost and energy-efficient mechanism to transmit thesquare-wave signal.

The load such as a personal computer will generally include a batterythat is able to power internal load circuitry. When the power adapter isdisabled by the load, the load may re-enable operation of the poweradapter by setting the secondary-side signal V2 low. Such a signalproduced by the load to turn the power adapter back on may betransmitted when the load enters a state of active use, or when itsbattery may need to be recharged. The disconnect switch Q2 on the dcside of the bridge rectifier 101 of the power adapter is configured todisconnect the ac mains voltage V1 from the power converter in responseto the secondary-side signal V2 from the load containing an independentpower source (e.g., battery V3). The independent power source in theload (e.g., battery V3) provides signal power for circuitry in the poweradapter that may be used to enable disconnecting or reconnecting the acmains voltage V1. For example, this signal power provided by the loadmay be employed to power circuitry such as the relaxation oscillator 102(e.g., the inverting Schmitt trigger U1 thereof) and the inverter INV1.The disconnect switch Q2 is configured to connect the ac mains voltageV1 to the power converter when the independent power source is unable toprovide the signal power.

The secondary-side circuit elements illustrated in FIG. 2 before thepower switch Q1 (i.e., before the gate of the power switch Q1) areassumed to be powered by the load coupled to the power adapter. If thebattery V3 in the load is discharged so that the load is inoperablewithout the power adapter, the relaxation oscillator 102 will not bepowered. As a result, no signal will be produced at the gate of powerswitch Q1, independent of the secondary-side signal V2. Accordingly,when a battery V3 in the load is fully discharged, a 12 volt bias willbe produced across the zener diode Z1, and the presence of ac mainsvoltage V1 will enable the disconnect switch Q2 to be turned on,enabling the battery V3 in the load to be recharged. To disable thedisconnect switch Q2 when the battery V3 in the load is charged, thesecondary-side signal V2 is set to a high value.

The power adapter illustrated in FIG. 2 provides a one nanofaradY-capacitor CY coupled across the high-voltage boundary between theprimary and secondary grounds. A smaller capacitor can be used if therelaxation oscillator frequency is increased. For instance, a 100picofarad Y-capacitor CY operating at 50 kilohertz (“kHz”) with a 2.2nanofarad value for the capacitor C2, and a 10 kilohm (“kΩ”) value forthe resistor R4 as relaxation oscillator 102 timing components may beemployed to advantage. When the personal computer or other load exits alow-power or zero-power operational mode, it sets the secondary-sidesignal V2 low. The relaxation oscillator 102 is then disabled, whichturns off the power switch Q1. Turning off the power switch Q1 allowsthe voltage of the zener diode Z1 to float back up to 12 volts, whichthen turns on the disconnect switch Q2, thereby enabling power to besupplied to the load. The gate voltage of the power switch Q1 isrepresented by Vg, and the ac mains voltage V1 coupled to the powerconverter powers the load such as a personal computer or a battery V3therein that may need to be charged.

Exemplary component values for circuit elements illustrated in FIG. 2are listed below.

Component Exemplary Values R1  2.8 megohms (“MΩ”) R3 a valuerepresenting a load and power converter losses is 105 kilohms (“kΩ”) R4330 kΩ R5  1 kΩ R6  10 MΩ R7  1 kΩ Z1  12 V CY  1 nanofarad (“nF”) C1  1nF C2 100 pF C3 330 nF Capacitors in voltage doubler 103  1 nF

Turning now to FIG. 3, illustrated is a graph showing selected voltagesof the power adapter of FIG. 2. The graph shows a gate voltage Vg of thepower switch Q1 via the resistor R7, which is on the primary side of thepower adapter after the oscillator signal. When the disconnect switch Q2is on, the power consumed by the exemplary adapter is 727 milliwatts(“mW”). When the disconnect switch Q2 is off, the power consumed by theexemplary adapter is 26 mW. The 26 mW power level is determined by thebleeder resistor R1. The 727 mW power level is the sum of the 26 mWconsumed by the bleeder resistor R1 and whatever quiescent load is onthe dc side of the bridge rectifier 101, represented here by theresistor R3. In many adapter designs, the equivalent value of theresistor R3 can be much higher due to inclusion of a burst mode ofoperation. If a burst mode of operation is included, the 727 mW numberwould be reduced. The 26 mW power level is determined by the size ofX-capacitor C3 and safety requirements that prescribe its rate ofdischarge.

When the ac mains voltage V1 is disconnected, the X-capacitor C3(parallel-coupled to the ac mains at an input of the power adapter)discharges through the bleeder resistor R1, zener diode Z1, and eitherdiodes D5, D10 or diodes D4, D7, depending on the polarity of thevoltage across X-capacitor C3. The bleeder resistor R1 discharges theX-capacitor C3 to a voltage below about 37 percent of its original valuein less than one second to satisfy a common safety requirement.

Turning now to FIG. 4, illustrated is a schematic drawing of a portionof an embodiment of a power adapter constructed according to theprinciples of the present invention. An active bleeder of the poweradapter is formed with a detection circuit 104 to detect the presence ofac mains voltage V1, and with an additional 12 volt supply formed with aresistor R8 and a zener diode Z2 derived from the same high-voltagecapacitor C1 that is used by the original 12 volt supply illustrated anddescribed with reference to FIG. 2. In addition, a bleeder switch (e.g.,a MOSFET) Q5 is included and a current-limiting bleeder resistor R9. Aninverter is formed by a switch (e.g., MOSFET) Q4 and a resistor R11,which is supplied by the added 12 volt supply. A coupling circuit isformed by a capacitor C7, a resistor R10, and a zener diode (e.g., a 12voltage zener diode) Z3. In the power adapter illustrated in FIG. 4, thebleeder resistor R1 illustrated in FIG. 2 is now increased to 100 MΩ.While the resistance of the bleeder resistor R1 is too high to rapidlydischarge the X-capacitor C3, the bleeder switch Q5 in series with thecurrent-limiting bleeder resistor R9 enables the parallel-coupledX-capacitor C3 to be discharged in acceptable time such as less than onesecond.

The bleeder switch Q5 is included in series with the current-limitingbleeder resistor R9 to actively bleed the X-capacitor C3 rapidly. Duringnormal operation, the added 12 volt supply keeps the switch Q4 on.Therefore, the bleeder switch Q5 is held in an off state. The poweradapter operates as before, except that the bleeder current issubstantially reduced during an off state. The no-load power is nowreduced to only 5 mW when the disconnect switch Q2 is off. When the acmains voltage V1 is disconnected, the two 12 volt supplies draw currentfrom the high-voltage capacitor C1 and the X-capacitor C3, causing aslow discharge thereof. The capacitor C7 acts as a coupling capacitor,causing the voltage change (e.g., decline) on the high-voltage capacitorC1 to appear at the gate of the switch Q4. When the voltage across thehigh-voltage capacitor C1 declines by about nine volts, the gate voltageVGQ4 of the switch Q4 declines from 12 volts to three volts. The switchQ4 then turns off, which turns on the bleeder switch Q5 and quicklydischarges the high-voltage capacitor C1 and the X-capacitor C3. Thedetection circuit detects the loss of the ac mains voltage V1 by sensinga voltage or a change thereof across the high-voltage capacitor C1. Thevoltage of the gate of the switch Q4 may, for example, exhibit a ripplevoltage in the range of five to nine volts at a high ac mains voltage(e.g., 264 volts root mean square (“V RMS”)) V1. At lower ac mainsvoltages V1, the ripple voltage at the gate of the switch Q4 will beproportionately less. The zener diode Z3 limits the gate voltage VGQ4 ofthe switch Q4 to about 12 volts. Thus, the detection circuit 104 isconfigured to detect the loss of ac mains voltage V1 by sensing avoltage across the X-capacitor C3 with a peak detector (e.g., diodes D4,D5 and high-voltage capacitor C1), and a change of voltage of the peakdetector is sensed employing the coupling capacitor C7 and the switchQ4.

Exemplary component values for circuit elements illustrated in FIG. 4are listed below.

Component Exemplary Values R1 100 MΩ R8 100 MΩ Z2, Z3  12 V R9  10 kΩR10 330 MΩ R11  4.7 MΩ C7 100 nF

Exemplary component values for other elements illustrated in FIG. 4 arethe same as those described with reference to FIG. 2. In accordancetherewith, like components in FIGS. 2 and 4 are referred to using thesame reference designations. As illustrated in FIG. 4, the X- andY-capacitors C3, CY, the bleeder resistors R1, R9, the bleeder switchQ5, the relaxation oscillator 102, the voltage doubler 103, thedetection circuit 104 and the disconnect switch Q2 form at least aportion of a power reducer for the power adapter.

Turning now to FIG. 5, illustrated is a graph showing selected voltagesof the power adapter of FIG. 4. The graph shows the gate voltage VGQ4 ofthe switch Q4 with normal operation of the power adapter at a high acmains voltage V1 of 264 volts alternating current (“VAC”). The gatevoltage VGQ4 at the gate of the switch Q4 exhibits a sawtooth ripplewaveform voltage with a peak-to-peak ripple voltage of about five voltsand a frequency double the ac mains frequency. The gate voltage VGQ4remains above a threshold voltage Vth of the switch Q4, which results inthe switch Q4 remaining on continuously during normal application of anac mains voltage V1. As a result, the bleeder switch Q5 remains offcontinuously. Accordingly, no power is dissipated in thecurrent-limiting bleeder resistor R9 during normal operation of thepower adapter when an ac mains voltage V1 is applied.

Turning now to FIGS. 6A and 6B, illustrated is a graph showing selectedvoltages of the power adapter of FIG. 4. The graph shows a voltage VC3(dotted line) across the X-capacitor C3 (see FIG. 6A), and a gatevoltage VGQ4 (dashed line) at the gate of the switch Q4 and a gatevoltage VGQ5 (solid line) at the gate of the bleeder switch Q5 (see FIG.6B), illustrating active-bleeder operation of the power adapter when theac mains voltage V1 is disconnected at time 0. When the ac mains voltageV1 is disconnected at time 0, the gate voltage VGQ4 at the gate of theswitch Q4 declines fairly slowly until it reaches the threshold voltageof the switch Q4 at approximately 0.4 seconds (“s”). At this time, theswitch Q4 turns off, causing the gate voltage VGQ5 of the bleeder switchQ5 to rise, turning on the bleeder switch Q5. The turn-on of the bleederswitch Q5 causes the rapid discharge of the X-capacitor C3 through thecurrent-limiting bleeder resistor R9, as illustrated by the voltage VC3across the X-capacitor C3 in FIG. 6A. Prior to the turn-on of thebleeder switch Q5 at approximately 0.4 seconds, the voltage VC3 acrossthe X-capacitor C3 only slowly decays through the resistors R1, R8, R10and the zener diodes Z1, Z2, Z3.

The active bleeder circuit described hereinabove can be used without adisconnect switch such as the disconnect switch Q2. However, thecircuits illustrated in FIGS. 2 and 4 share common circuit elements, andeach may reduce no-load losses by similar orders of magnitude.Accordingly, both circuits can be advantageously used to reduce no-load(or light load) losses.

In a further embodiment, a start-up circuit produces an initial biasvoltage for the power adapter and also discharges a capacitor such as anX-capacitor coupled across the ac mains upon loss of ac mains voltage.Upon a loss of the ac mains voltage, a residual voltage can remainacross a capacitor coupled to input terminals of a power adapter, whichpresents a possible safety risk to an end user that might unplug thepower adapter and touch exposed pins of its ac plug. Accordingly, acircuit element such as a resistor is generally included in the poweradapter to discharge the capacitor. However, as described previouslyhereinabove, such a resistor when continuously coupled to the capacitorproduces undesirable power dissipation within the power adapter. In anembodiment, a resistor-capacitor time constant is employed to initiatedischarge of the capacitor upon a loss of the ac mains voltage.

Turning now to FIG. 7, illustrated is a schematic drawing of anembodiment of portions of a power adapter formed with start-up circuit760 and crowbar circuit 750 configured to discharge a capacitor C20(e.g., an X-capacitor) coupled across the ac mains. An ac mains voltageV1 is coupled to a diode bridge 701 to produce a rectified input voltageV_(in). The capacitor C20 is coupled across dc terminals of the diodebridge 701. The start-up circuit 760 formed with a switch (e.g., aMOSFET) Q10 is coupled across the capacitor C20 and includes a resistorR10 series-coupled to a non-control terminal (e.g., a source terminal)of the switch Q10 to provide a substantially constant-current biasvoltage source for the power adapter. In an embodiment, the start-upcircuit 760 may be a double-diffused metal-oxide semiconductor (“DMOS”)start-up circuit (or cell) wherein the switch Q10 is a DMOS transistor.The resulting bias voltage is coupled to a bias voltage filter capacitorC40 through a diode D17 to produce a bias voltage V_(dd).

When a switching action of the power converter is initiated, ahigh-frequency internal bias voltage source 740 is rectified by a diodeD18 and coupled to the bias voltage filter capacitor C40. A resistor R40represents a bias load of the power adapter that is powered by the biasvoltage V_(dd). When the bias voltage V_(dd) is produced by thehigh-frequency internal bias voltage source 740, it is generally severalvolts higher than when the bias voltage V_(dd) is produced by thestart-up circuit 760. A zener diode Z10 (e.g., a ten volt zener diode)is operable to disable the start-up circuit 760 when the bias voltageV_(dd) is greater than a threshold value, such as 12 volts, therebyeliminating unnecessary power dissipation in the power adapter. Thus,the start-up circuit 760 provides the bias voltage V_(dd) when the biasvoltage V_(dd) provided by the high-frequency internal bias voltagesource 740 of the power adapter is less than a threshold value.

The ac mains voltage V1 is also coupled to a crowbar circuit 750 thatturns on the start-up circuit 760 upon loss of the ac mains voltage V1.The crowbar circuit 750 is formed with resistor-capacitor network 710that is coupled to a capacitor C30 through a diode bridge 720. A crowbarswitch Qcb (e.g., an npn bipolar transistor) is coupled to the capacitorC30 through a diode Dcb. When sufficient ac mains voltage V1 is present,the resulting voltage produced across the capacitor C30 is sufficient toturn off the crowbar switch Qcb. However, when the ac mains voltage V1is disconnected or is of insufficient amplitude, the crowbar switch Qcbis enabled to conduct by a resistor Rcb coupled between a controlterminal and a non-control terminal (e.g., the base and collector) ofthe crowbar switch Qcb. The resulting conduction of the crowbar switchQcb turns on the start-up circuit 760 to discharge the capacitor C20. Adiode D17 isolates the bias voltage filter capacitor C40 from thecrowbar circuit 750.

Thus, in an embodiment, a bleeder resistor for discharging a capacitor(e.g., an X-capacitor) is replaced with a start-up circuit such as aDMOS start-up circuit that is ordinarily employed to just provide aninitial bias voltage for a power adapter or other electronic circuit. Acrowbar circuit turns on a switch to bleed charge (via the start-upcircuit) from the capacitor upon loss of ac mains voltage. A switch anda source resistor are configured as a substantially constant-currentbias voltage source. The start-up circuit includes a zener diode to turnoff the switch when the power adapter initiates its switching action. Aslong as ac mains voltage is above a threshold level, such as a root-meansquare (“RMS”) value of the ac mains voltage greater than 100 Vac for anominal 120 Vac mains, and an internal bias voltage is present, thestart-up circuit is turned off. If the ac mains voltage is lost, thenthe start-up circuit is turned on by a crowbar circuit. The start-upcircuit is thus used for two purposes. One is to start up an initialbias voltage, and the other is to bleed off charge from a capacitorcoupled across ac input terminals of the power adapter when there is aloss of ac mains voltage supplied to the power adapter or if the acmains voltage drops too low to turn on the start-up circuit.

Thus, a start-up circuit for a power adapter and method of operating thesame have been introduced herein. In one embodiment, the power adapterincludes a start-up circuit (e.g., a DMOS start-up circuit) configuredto provide an initial bias voltage for the power adapter. The poweradapter also includes a crowbar circuit configured to turn on thestart-up circuit upon loss of an ac mains voltage supplied to the poweradapter. The start-up circuit is configured to provide the initial biasvoltage when a bias voltage of the power adapter is less than athreshold value. The start-up circuit is operable to discharge acapacitor (e.g., an X-capacitor) coupled across input terminals of thepower adapter. The start-up circuit includes a switch (e.g., a MOSFET)and resistor coupled (e.g., series-coupled to a source of the MOSFET)thereto configured to provide a substantially constant-current biasvoltage source. The start-up circuit is coupled to the ac mains voltagevia a diode bridge and employs the ac mains voltage to provide theinitial bias voltage. The crowbar circuit is configured to turn on thestart-up circuit when the ac mains voltage falls below a thresholdlevel. The crowbar circuit includes a resistor-capacitor network coupledto the ac mains voltage via a diode bridge. The crowbar circuit includesa switch (e.g., a bipolar transistor) coupled to a capacitor through adiode, and is isolated from a bias voltage filter capacitor by a diode.

Those skilled in the art should understand that the previously describedembodiments of a power adapter including circuits to reduce no-load (orlight load) losses and related methods of operating the same aresubmitted for illustrative purposes only. For example, in a furtherembodiment, a power adapter that uses a half-wave bridge instead offull-wave bridge can use techniques described herein. For example,full-wave bridge 101 illustrated in FIGS. 2 and 4 could be replaced witha single diode, eliminating the need for the diode D4. While a poweradapter employing a power converter including circuits to reduce no-load(or light load) losses has been described in the environment of a powerconverter, these processes may also be applied to other systems such as,without limitation, a power amplifier or a motor controller.

For a better understanding of power converters, see “Modern DC-to-DCPower Switch-mode Power Converter Circuits,” by Rudolph P. Severns andGordon Bloom, Van Nostrand Reinhold Company, New York, N.Y. (1985) and“Principles of Power Electronics,” by J. G. Kassakian, M. F. Schlechtand G. C. Verghese, Addison-Wesley (1991). For related applications, seeU.S. Patent Application Publication No. 2008/0130321, entitled “PowerConverter with Adaptively Optimized Controller and Method of Controllingthe Same,” to Artusi, et al., published Jun. 5, 2008, U.S. PatentApplication Publication No. 2008/0130322, entitled “Power System withPower Converters Having an Adaptive Controller,” to Artusi, et al.,published Jun. 5, 2008, and U.S. Patent Application Publication No.2008/0232141, entitled “Power System with Power Converters Having anAdaptive Controller,” to Artusi, et al., published Sep. 25, 2008. Theaforementioned references are incorporated herein by reference in theirentirety.

Also, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the processes discussed above can be implemented indifferent methodologies and replaced by other processes, or acombination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods, and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A power adapter, comprising: a start-up circuitconfigured to provide an initial bias voltage for said power adapter;and a crowbar circuit configured to turn on said start-up circuit uponloss of an ac mains voltage supplied to said power adapter.
 2. The poweradapter as recited in claim 1 wherein said start-up circuit comprises aswitch and resistor coupled thereto configured to provide asubstantially constant-current bias voltage source.
 3. The power adapteras recited in claim 2 wherein said switch is a metal-oxide semiconductorfield-effect transistor and said resistor is series-coupled to a sourceterminal thereof.
 4. The power adapter as recited in claim 1 whereinsaid start-up circuit employs said ac mains voltage to provide saidinitial bias voltage.
 5. The power adapter as recited in claim 1 whereinsaid start-up circuit is coupled to said ac mains voltage via a diodebridge.
 6. The power adapter as recited in claim 1 wherein said crowbarcircuit is configured to turn on said start-up circuit when said acmains voltage falls below a threshold level.
 7. The power adapter asrecited in claim 1 wherein said start-up circuit comprises adouble-diffused metal-oxide semiconductor start-up cell.
 8. The poweradapter as recited in claim 1 wherein said crowbar circuit comprises aresistor-capacitor network coupled to said ac mains voltage.
 9. Thepower adapter as recited in claim 1 wherein said crowbar circuitcomprises a diode bridge coupled via a resistor-capacitor network tosaid ac mains voltage.
 10. The power adapter as recited in claim 1wherein said crowbar circuit is isolated from a bias voltage filtercapacitor by a diode.
 11. The power adapter as recited in claim 1wherein said crowbar circuit comprises a switch coupled to a capacitorthrough a diode.
 12. The power adapter as recited in claim 11 whereinsaid switch as a bipolar transistor.
 13. The power adapter as recited inclaim 1 wherein said start-up circuit is operable to discharge acapacitor coupled across input terminals of said power adapter.
 14. Thepower adapter as recited in claim 12 wherein said capacitor is anX-capacitor.
 15. The power adapter as recited in claim 1 wherein saidstart-up circuit is configured to provide said initial bias voltage whena bias voltage of said power adapter is less than a threshold value. 16.A method, comprising: providing an initial bias voltage for a poweradapter with a start-up circuit; and turning on said start-up circuitupon loss of an ac mains voltage supplied to said power adapter.
 17. Themethod as recited in claim 16 wherein said start-up circuit provides asubstantially constant-current bias voltage source.
 18. The method asrecited in claim 16 wherein said turning on said start-up circuitcomprises turning on said start-up circuit when said ac mains voltagefalls below a threshold level.
 19. The method as recited in claim 16furthering comprising discharging a capacitor coupled across inputterminals of said power adapter.
 20. The method as recited in claim 16wherein providing said initial bias voltage comprises providing saidinitial bias voltage when a bias voltage of said power adapter is lessthan a threshold value.